Such couplings, which are also designated as Johnston couplings, are generally used in vacuum-insulated piping systems which are used for transporting cryogenic media. Due to the elements of the coupling, which are expediently of double-walled and vacuum-insulated design, greater spatial distance between the connecting point of the two outer coupling elements and the point at which the cryogenic medium passes over from the one line into the other line is provided. This is intended to reduce icing of the coupling point and losses of the cryogenic medium due to vaporization. In such a coupling, the coupling plug is also designated as “male part” and the associated coupling socket as “female part”.
A plug-in coupling for the inner pipes of a vacuum-insulated pipeline is known from DE-A-3142702, in which plug-in coupling the inner pipes are fitted together like a male/female connection. In the case of this connection, liquid can escape at the front-end connection of the pipes and can enter the annular gap between the pipes of the plug-in connection and vaporize there. Since this annular gap is sealed at its other end, a pressure cushion which prevents further escape of liquid from the inner pipe builds up. However, this pressure cushion can only build up in the case of an arrangement inclined at least relative to the horizontal, in particular a vertical arrangement, and then only if the male part is inserted into the female part from above.
A plug-in connection for line pipes like a male/female connection for transporting low-temperature media is known from DE-U-84 32 062, in which plug-in connection the inner plug part (male part) is made of a metal having a low coefficient of expansion and the outer plug part (female part) is made of a metal having a normal coefficient of expansion. The inner plug part is made of a steel containing more than 30% nickel, whereas the outer plug part is made of brass, copper or high-grade steel. In this arrangement, the outer coupling part of the coupling parts fitted one inside the other therefore shrinks onto the inner coupling part when the cryogenic medium flows through the line pipe. In this way, the annular gap required for fitting the coupling parts one inside the other is sealed and escape of liquid or vapor is restricted to the greatest possible extent. However, a precondition for this is that those surfaces of the coupling parts which face one another have extremely low roughness. Nonetheless, 100% sealing of the annular gap cannot be achieved, such that there is still the risk of icing in the “warm” region of the plug-in coupling and therefore a loss of cryogenic liquid occurs.
DE 41 07 652 A1 describes a device for coupling pipelines for cryogenic media, in which device the ends of two vacuum-insulated pipelines to be connected are provided with end pieces which make possible a coupling like a male/female connection. In this coupling, the annular gap between the male part and the female part is closed at both ends by a seal.
A plug-in connection for vacuum-insulated line pipes which consist of an inner pipe and an outer pipe and are intended for transporting liquefied gases is known from US 2002/0089177 A1, the end regions of which plug-in connection can be fitted one inside the other like a male/female connection. Each line pipe has a safety valve which is welded in place in the outer pipe and opens in the event of an excess pressure in the annular space between the inner pipe and the outer pipe. The annular gap between the male part and the female part is not sealed at the end.